This invention relates generally to processing of dielectric and, in particular, ferroelectric materials and more particularly to processing of such materials that have useful properties in semiconductor applications.
As it is known in the art, in modern digital computers one of the most commonly used circuits are semiconductor memory circuits. Semiconductor memory circuits generally include several types, random access memory types (RAM), electrically alterable read only memories (EAROM), electrically programmable read only memories (EPROM), and read only memories (ROM). In general, such semiconductor memories are fabricated with transistors, diodes, capacitors, and the like to provide memory elements and associated control circuitry as required. Generally the latter two types EPROM and (ROM) are designed to be programmed only once, the programmable read only memories (PROMs) proms being programmed by electrical means such as breaking electrical paths, whereas the ROMs being programmed during the fabrication process of the device. The first type (i.e. the random access memory) which may be a static or dynamic memory type is designed to be programmed and reprogrammed an indefinite number of times generally on the order of 10.sup.11 times. The RAM is generally used as the basic high speed memory in digital computers. RAMs are characterized as having read cycle times (i.e. data retrieval) and write cycle times (i.e. data storage) of about the same duration.
One problem with these memories is, however, the memories are generally volatile. That is, during loss of power the memory are generally lost. In many applications, sudden loss of information in these memories is undesirable.
There are several approaches which have been used to overcome these problems. Magnetic core memories and more recently plated wire memories are used to insure non-volatility. The plated wire or magnetic core memory approach is undesirable since they are each relatively large and of low density of data storage compared to semiconductor memories. They are also very expensive to build, are heavy, and require the need for several high power, power supplies. Another approach has been to provide battery backup to the random access memories. The problem with the battery backup is that it is generally cumbersome and adds increased cost to the circuit.
The EAROM is likewise not a suitable solution since it is not a random access memory. The electrically alterable read only memory is not random access because the write cycle time for such a device is generally an order of magnitude or more greater than the read cycle time making this device unsuitable for random access memory applications.
One approach which is currently being developed to provide a non-volatile random access memory is based on the use of ferroelectric materials. Ferroelectric materials such as perovskite materials of the form ABO.sub.3 have many useful properties which can be exploited to provide a variety of electronic and opto-electronic devices such as the aforementioned non-volatile memories, as well as optical switches, optical modulators, pyroelectric detectors, integrated circuit artificial intelligence neural networks, and a variety of micromechanical devices.
Returning to the aforementioned non-volatile memory applications, in these applications ferroelectric films are provided over a semiconductor substrate including generally a large number of transistors.
In one type of memory element, the perovskite material film is disposed between a pair of conductors and is used as a polarizable dielectric for a capacitor. Depending upon the direction in which the voltage is applied to the capacitor, the direction of polarization of the ferroelectric dielectric in the capacitor will vary. This variation in polarization, therefore, can be exploited in a memory to provide a logic one or a logic zero (i.e. digital data bit) stored in the particular location of the memory. Since a principal driving force in all semiconductor memory applications is to increase the density of bits (or i.e. memory cells which can be located in a given area of semiconductor memory), it is generally desirable to have the capacitors formed adjacent to and over the transistors used to address individual memory elements of the semiconductor memory.
One ferroelectric material which is useful in the above-mentioned applications is the material known as lead zirconate titanate having the general chemical formula as (Pb.sub.x Zr.sub.y Ti.sub.1-y O.sub.3) which will hereinafter be referred to as PZT. PZT is a multicomponent material (i.e. a ternary alloy of lead oxide, zirconium oxide, and titanium oxide). Several deposition techniques can be used to provide PZT films. One such technique is a solution gelation technique (sol-gel). One type of sol-gel process is described in a paper entitled "Sol-Gel Processing of PbTiO.sub.3, PbZrO.sub.3, PZT, and PLZT Thin Films" by Budd, et al., Proceedings of the British Ceramic Society, Vol. 36 (1985), pages 107-121.
A second sol-gel technique known in the art is described in a paper entitled "Preparation of Pb(Zr,Ti)O.sub.3 Thin Films by Sol Gel Processing: Electrical, Optical, and Electro-optic Properties" by Yi, et al., Journal of Applied Physics, Vol. 64, No. 5 (September 1988), pages 2717-2724.
It is also known that, in order to exploit thin film PZT for integrated circuit applications, it is necessary to selectively pattern the PZT using manufacturable lithographic processes. Several other techniques for patterning PZT are available, such as wet chemical etching with HF solutions and ion milling, but each has serious drawbacks. Undercut of the photoresist masking pattern with wet chemical etches makes them unsuitable for state of the art, high packing density microcircuits. In addition, when the PZT crystallizes into mixed pyrochlore and perovskite phases, it becomes extremely difficult to pattern the PZT without severely undercutting photoresist masks because the etch rates of the two phases in common wet etchants may be dissimilar. Ion milling (or sputter etching) suffers from a lack of selectivity to underlying layers and excessive substrate heating (which can degrade PZT electrical properties) associated with attempting to achieve high etch rates required for efficient ion milling in a manufacturing environment.